ADP-ribosylation, in which the ADP-ribose moiety of NAD is transferred to a target protein, is catalyzed by a family of bacterial toxins and mammalian enzymes. Some toxin transferases appear to be responsible for the diseases caused by the bacterium. The mammalian ADP-ribosyltransferases (ARTs) are located both within the cell and on the cell surface, sometimes, linked through a glycosylphosphatidylinositol anchor (ART1). Other mammalian ADP-ribosyltransferases (ART5) apppear to be secreted. A family of the mammalian enzymes has been cloned in the laboratory. ART2a (RT6.1) and ART2b(RT6.2) are NAD glycohydrolases (NADases) that are linked to T lymphocytes by glycosylphosphatidylinositol (GPI) anchors. Although both mature proteins possess three conserved regions (I, II, III) that form the NAD-binding site and differ by only ten amino acids, only ART2b is auto-ADP-ribosylated and only ART2a is glycosylated. To investigate the structural basis for these differences, wild-type and mutant ART2a and ART2b were expressed in rat mammary adenocarcinoma (NMU) cells and released with phosphatidylinositol-specific phospholipase C. All mutants were immunoreactive NADases. Arginine 204 (Arg204), amino-terminal to essential glutamate 209 in Region III, is found in ART2b, but not ART2a. Replacement of Arg204 in ART2b with lysine, tyrosine, or glutamate abolished auto-ADP-ribosylation. Unlike wild-type ART2a, ART2a(Y204R) was auto-ADP-ribosylated. The tryptophan mutant ART2b(R204W) was auto-ADP-ribosylated and exhibited an enhanced NADase activity compared to the wild-type ART2b. Incubation with NAD with auto-ADP-ribosylation decreased the NADase activities of wild-type ART2b and ART2b(R204W), whereas activity of ART2b(R204K), which is not auto-modified, was unchanged by NAD. Facilitation of auto-ADP-ribosylation by tryptophan 204 suggests that the hydrophobic amino acid mimics an ADP-ribosylated arginine and may be involved in the regulation of ART activity. . Thus, Arg204 in ART2b serves as a regulatory switch whose presence is required for additional auto-ADP-ribosylation and regulation of catalytic activity.